1. Field of the Invention
The present invention relates to a semiconductor component; and more particularly, to a package for power semiconductor circuit components. Although the present invention is suitable to many high power components, it is especially suited for use with a substrate MOS Controlled Thyristor (MCT); and will be particularly described in that connection.
2. Description of Related Art
Designers of today's electronic circuitry and power supplies strive for miniaturization. As higher power density is demanded, advanced components and packaging capabilities must be produced. Recently developed materials and components, such as the substrate MOS Controlled Thyristor (MCT), MCT, and insulated gate drive bipolar transistor (IGBT), significantly contribute to the increase in power density.
A power component package must enable high thermal dissipation and high current conductance from a very small surface area. An ultra high power semiconductor device such as a substrate MCT has recently been developed such that a small chip (0.750".times.0.750") can conduct a 2000 ampere current and a power density of 550 watt/cm.sup.2. It's application can simplify many power conversion circuit designs. For example, this chip can replace a large number of field-effect transistors (FETs) connected in parallel and series and used as a high power switch. An FET is a semiconductor device whose operation depends on the use of an applied electric field to control device current. Metal oxide semiconductor FETs (MOSFETs) are often arranged in parallel and series in integrated circuits as switches.
A chip such as the substrate MCT requires a package capable of removing high levels of current density and thermal density. A package design has been proposed that utilizes large gauge aluminum wirebonds (0.01 inches or 0.02 inches in diameter) to connect in parallel anode cells on the chip surface to a surrounding copper surface. The chip is mounted on an electrically isolated second copper surface that acts as the cathode.
Because the aluminum wirebonds extend from various points on the chip to the surrounding copper surface, the wirebond lengths vary. This large length variation in parallel wirebonds causes a greater proportion of the current from the chip to conduct through the Short wirebonds. This brings about a domino effect failure, starting with the short wirebonds burning open.
In addition, the length of longer wirebonds may exceed its mechanical reliability limit. As the size of the MCT is approximately 0.750".times.0.750", the wirebonds connecting the middle anode cells to the copper surface are at least 0.375 inches long. The mechanical reliability limit of the wirebond is a maximum of 0.200 inches.
Longer wires also cause greater voltage drops along the wire. As the resistance of a wire depends on its length, the longer wires will be more resistant. Due to Ohm's Law, the voltage drop along such wires is greater.
Conductance of the current from the MCT chip requires a large number of wirebonds. For example, the current rating of a 0.02 inch diameter aluminum wirebond is 20 amperes. For a high current application of 2000 amperes, at least 100 such wirebonds must be used. The area surrounding a typical chip may be insufficient to bond the required number of wirebonds.
In light of the foregoing, there is a need for a package for a power semiconductor device that utilizes shorter uniform length wirebonds, has sufficiently large wirebonding area, and enables high thermal dissipation and high current conductance with minimal resistance.